The present invention relates to screw caps of thermoplastic material, particularly intended for the closure of glass containers used in the food industry, of the type comprising a horizontal end wall, a cylindrical skirt provided with internal threading for the screwing of the cap onto the externally-threaded mouth of a container, and sealing means carried by the internal surface of the horizontal end wall and adapted to cooperate with the mouth of a container to form a seal between the inside of the container and the cap when the cap is screwed onto the container, said cap being provided on the outer face of said horizontal end wall with a thin covering layer of a composite laminated material which is adapted to form a barrier against the passage of gases through said wall said thin covering layer being formed by a disc incorporated in the horizontal end wall of the cap during the injection moulding of the cap.
A screw cap of the above mentioned type is known from EP-A-0 239 238, which suggests that the disc forming the covering layer should extend over substantially the entire area of the outer surface of the horizontal end wall of the cap. The cap may be provided with a relatively impermeable sealing gasket to reduce the permeability of the cap.
The use of a disc having a diameter which is equal to (or smaller than) the diameter of the circular outer face of the horizontal end wall of the cap does not allow to obtain caps which are satisfactory from an aesthetic point of view, because in mass production a slight eccentricity between the disc and the circular horizontal end wall of the cap is practically unavoidable. Said eccentricity is particularly noticeable when the disc carries a printed decoration or information.
Moreover if a cured plastisol liner, which is relatively permeable, is used as sealing means, passage of gas may occur through the upper portion of the cylindrical skirt of the cap which is adjacent to the horizontal end wall of the cap.
A method for the manufacture by injection moulding of thermoplastic material into the cavity of a mould of a screw cap of the type mentioned in the first paragraph of the present specification is known from EP-A-0 239 238.
According to said method, prior to the injection, the disc is inserted into the cavity of the mould and is held in position therein, so that when the thermoplastic material is injected the disc becomes moulded into and permanently bonded to the outer surface of the horizontal end wall of the cap.
According to the teaching of said document the disc is held in position against the wall of the cavity corresponding to outer face of the horizontal end wall of the cap by providing a vacuum between the disc and the cavity. This is achieved by providing one or more vacuum ports, attached to a vacuum pump, in the region of the cavity where the disc is to be held.
Said vacuum ports are subject to be occluded by the thermoplastic material injected into the cavity of the mould if the material is injected in the absence of a disc or in the presence of a defective disc, which is subject to be bored under the pressure of the injected fused material.
Moreover the provision of vacuum passages in the mould and of vacuum pump connected to said passages increases the costs of the mould.